It is commonly known that materials expand and contract with changes in temperature. A commonly used quantifier of this natural phenomenon is the coefficient of thermal expansion or CTE. Generally, CTE can be thought of as a ratio of the change in length of a line segment in a body per unit of temperature change to its length at a reference temperature. When a material is heated, its linear dimensions increase approximately in proportion to the temperature. Over moderate changes in temperature, the length of a material changes by an amount:ΔL=Lo α(ΔT)where Δ (delta) indicates “change in”, T is temperature (in degrees Fahrenheit), and L is length (in inches). The constant α is called the coefficient of thermal expansion (typically measured in 10−6 in./in. ° F.), and Lo is the initial length, before expansion (in inches). Consideration of the CTEs of materials used in aerospace applications is important as high temperature swings may occur both in the manufacturing process as well as when the materials are put in use. Under ideal circumstances, the part being manufactured and the tool used to manufacture the part would be comprised of a similar or the same material because the CTEs would match, and thus the tool and the part being formed would preferably expand and contract at the same rate with any temperature changes. However, this commonality of CTEs is not always practically achievable.
Dissimilar materials typically have different CTEs, and the union of dissimilar materials can impart a residual thermal loading effect between the materials, as they will expand and contract at different rates. A mismatch in the CTEs of the part and the tooling used to form the part will often result in complications for maintaining dimensional accuracy. For example, if a carbon/epoxy part is made using an aluminum tool, that tool may grow as much as 3-4 times as the carbon/epoxy pail during the manufacturing process, accordingly altering the desired dimensions of the part being formed. The carbon/epoxy part will likely cure hard, and when the tool and part are cooled, the aluminum tool may potentially contract (cool back) 3-4 times more than the carbon/epoxy part being formed. This discrepancy in the contraction and expansion due to cooling typically causes the part to experience dimensional mismatches due to the residual thermal loading effect. However, despite these disadvantages, it is often necessary to use different materials for tooling and the parts formed using the tooling in aerospace manufacturing applications because of other advantages that the tooling materials may provide.
In the past, some aerospace applications have utilized carbon/epoxy tooling to form composite parts. It should be appreciated that the thru-plane CTE of a carbon/epoxy mandrel is higher than the in-plane CTE of a carbon/epoxy laminate. As such, the thermal expansion of a mandrel is typically higher than that of a laminate, but this physical property may contribute to ironing out any imperfections in the pail being formed. Accordingly, wrinkling of the part may be minimized upon autoclaving the tooling and part during the manufacturing process. However, problems occur upon cooling the mandrel and the part. Once the combination is cooled, the resultant structure of the part is still smooth (i.e., had little to no visible wrinkling), however, the part experiences undesirable sagging, for example, from the mandrel.
While carbon/epoxy may sometimes be used for tooling, much of the tooling for production of composite aircraft parts is typically constructed from invar alloy, an alloy of iron and nickel including some carbon and chromium. Invar alloy has a similar expansion rate as some composite materials, such as carbon/epoxy, and therefore maintains a close tolerance for dimensional control. While invar tools typically produce a desirable final skin shape for the part being manufactured (i.e., not as much sagging as when carbon/epoxy mandrels are utilized), often the skin laminate is not as preferable in that wrinkles sometimes appear in the part that is formed using the invar tooling. This wrinkling typically results because parts cured on invar tooling may not grow enough to eliminate the bulk factor wrinkles. Accordingly, there exists a need in the art to provide a tool that is able to expand the part enough to address potential problems with wrinkling while also being able to iron out the resultant structure.
One type of tooling is a mandrel. Mandrels produced from carbon/epoxy using traditional methods typically experienced more growth during the tooling process than is desirable, particularly during the autoclave portion of the manufacturing process. The parts formed had an enlarged circumference and radius because the CTE associated with the tooling being used continued to be higher than was preferable due, in part, to the hat radii opening up because of the difference in thru-plane and in-plane CTEs of the carbon/epoxy.
The resultant enlarged circumference and higher CTE of the tooling formed parts that sagged from the mandrels upon conclusion of the autoclaving process. Furthermore, the growth required that shims were needed to hold the part in place during the trimming process. Accordingly, it was determined that use of traditional carbon/epoxy mandrels produced good quality laminates; however the final skin shape may not be as desirable because the radius and circumferential dimensions of the part formed may be too large for use in one-piece barrel aerospace applications.
To address these problems caused by use of composite tooling, it was thought that a smaller tool could be designed such that the tool could experience growth during the manufacturing process. In essence, the shape of tool could be modified so that when the tool expands, it expands to the proper location. However, modifying the size of the tool was not preferable as the expansion of the tool affects the dimensions and overall shape of the part being formed.
Thus, there exists a need to understand what causes the higher-than-expected thermal growth in composite tooling and to determine ways to control the thermal expansion of these tools during the manufacturing process in order to reduce the likelihood that the parts being; formed suffer from sagging or wrinkling. Accordingly, there is a need to control the thermal expansion of composite tooling during an aerospace manufacturing process in order to eliminate the sagging condition experienced by parts formed using such tooling.